Biodiesel is a sustainable biofuel derived from vegetable and/or animal fats. Advantageously, it is available from renewable resources and has a less negative environmental impact than fossil fuels.
Traditionally, biodiesel has been produced by transesterification of animal or vegetable fats using a catalyst such as sodium or potassium hydroxide. The transesterification reagent is typically a short chain alcohol such as methanol or ethanol, or a simple aliphatic ester of such an alcohol.
To overcome the drawbacks usually associated with the use of conventional chemical catalyst in biodiesel production, Sake and Kusdiana (Fuel 80: 225-231 2001) proposed a non-catalytic process carried out in supercritical methanol. The main advantages of using supercritical alcohols, especially methanol, over conventional catalyzed methods are faster reaction rate, easy product purification and the possibility of using inexpensive, unrefined feedstock (Ind. Eng. Chem. Res. 35: 607-612 1996; Biores. Technol. 96: 769-777 2005). However, the high critical temperature of methanol renders the process extremely energy intensive.
In another approach, biodiesel can be produced via enzymatic catalysis using a lipase enzyme. This strategy has shown promising results, primarily due to the high selectivity and mild operative conditions of the enzymatic reaction (J. Biotechnol. 119: 291-299 2005). In addition, in this process there is no need for feedstock purification, as lipase is capable of transesterifying triglycerides as well as esterifying free fatty acids present in the feedstock (J. Am. Oil Chem. Soc., 73: 1191-1195 1996). The main disadvantage of using lipase is its high cost. Accordingly, it must be used in immobilized form that allows its repeated reuse (J. Am. Oil Chem. Soc., 78: 585-588 2001). Besides reusability, immobilized lipase has been found to have enhanced activity and stability compared to the free form (Ind. Eng. Chem. Res., 43: 1568-1573 2004; Biores. Technol., 100: 5126-5131 2009).
Meanwhile, there remains a demand for inexpensive feedstocks for biodiesel production. This search is driven by the high cost of straight vegetable oils currently used as feedstocks. It has been suggested that use might be made of animal fats that are unsuitable for consumption as food. Such fats, however, generally have high melting points and therefore attempting to liquefy them and then use lipase to catalyse transesterification would lead to denaturation of the lipase. One countermeasure is to dissolve the fat in an organic solvent prior to effecting the enzymatic transesterification reaction (Biofuels Bioprod. Bioref., 1: 57-66 2007; Biotechnol. Adv., 27: 398-408 2009). However, the use of organic solvents has its own disadvantages, such as the introduction of new requirements for solvent separation and recovery.
Supercritical carbon dioxide (SC-CO2) has been used as a carrier for the lipase-catalysed production of biodiesel from vegetable oils (Fuel 86: 2650-2659 2007). However, use of SC-CO2 is expensive and therefore this process has not been widely applied. Furthermore, modification of the process to deal with animal fats, which unlike vegetable oils are solids rather than liquids, has not previously been reported.
There is accordingly still a requirement for alternative processes for producing biodiesel, which are procedurally convenient to carry out and which can be carried out on an industrial scale in an economically feasible manner.
One possible source of an animal fat feedstock for biodiesel production would be fat extracted from animal meat. Meat forms a major part of the human diet. Extraction of fat from animal meat prior to its use as a foodstuff is potentially beneficial to health. However, removal of animal fats is not always trivial, particularly when it is present in the meat in the form of intramuscular fat (IMF, commonly known as “marbled fat”) rather than as visible fat on the meat surface. For example, meat processors are capable of physically trimming surface fat from meats that have high external fat content, such as beef, so as to produce lean meat. However, marbled fat cannot be removed by such methods. Thus, although low-fat lean meat is readily available from meats such as beef, low-fat meat from sources having high IMF content, for example lamb, is not readily available. Methods to remove lamb fat from lamb meat, such as by solvent or supercritical fluid extraction, are typically prohibitively expensive at present and so are not carried out on an industrial scale. Accordingly, it would also be desirable to render economically feasible a process for generating lean lamb meat.